Precipitation and Dissolution of Sulfate Minerals: Mechanisms and Kinetics Studied with Scanning Force Microscopy

D. Bosbach Institut für Mineralogie, Corrensstr. 24, 48149 Münster

Precipitation and dissolution of minerals from aqueous solutions are important processes in near surface regions of the earth. Rates of mineral precipitation and dissolution are usually derived from bulk experiments, where different mechanisms of different crystallographic surfaces are potentially convoluted. Although such experiments are very important, the contribution of individual mechanisms and their kinetics often cannot be deciphered unambiguously. However, predicting rates of such geochemical processes requires that individual mechanisms are considered. Scanning Force Microscopy allows imaging of crystal growth and dissolution processes in-situ down to molecular scales, thus revealing individual mechanisms and their kinetics. However, in early SFM designs the sample was scanned relative to the SFM tip, which implies certain limitations regarding the sample size and its weight. Recently developed SFMs scan the tip relative to the sample. Therefore, since the sample is kept at a fixed position there are almost no sample weight and size limitations. Further, a suitable sample holder can be used to image almost every crystallographic surface. In-situ growth and dissolution experiments with SFM on the sulfate minerals gypsum, anhydrite and barite - celestite - anglesite clearly show that different growth modes occur as a function of the saturation state. Further, the influence of organic growth inhibitors (e.g. phosphonic acid derivatives) on the microtopography of certain crystallographic surfaces and the related growth rate reveal some fundamental aspects on the actual inhibiting mechanism.